create empty /etc/logstash/conf.d directory [fixes #2188]

Fixes #2189
reinstate standalone flag and remove .bundle in vendor task

Fixes #2184
remove explicit references to ruby implementation and version

Fixes #2184
remove spool codec from default_plugins

Fixes #2167
abort artifact if plugin installation fails

Fixes #2186
short circuit simple json string generation and memoize iso8601 string

Fixes #2179
timestamp.to_iso8601 works for all platforms and faster than time format parsing

Fixes #2179